MjpegServer uses the timeout to generate keep-alives to any clients
(which helps detect disconnects and avoid stale client threads).
CvSink GrabFrame now defaults to a timeout, but the timeout can be
changed by the user, or the old no-timeout version is now available
as GrabFrameNoTimeout.
This makes them available for both UsbCamera and HttpCamera / AxisCamera.
To avoid virtual functions in the public-facing interface, move the
implementation of the camera settings functions to the core library.
This takes hosts (IP or DNS name) rather than URLs, making it easier
to use.
Also add more overloads to resolve ambiguities encountered when using
std::string and const char*, and also add overloads for
std::initializer_list<T> so braced initializer lists can be used.
